In automotive power steering systems, the flow of fluid delivered to the power steering equipment must be controlled in accordance with the requirements of the steering gear. In these systems, a pump, which is the source of the delivered fluid is provided with a flow control apparatus, typically a flow control valve. Typically, hydraulic flow is controlled by a piston that moves reciprocally in a flow control cylinder to open and close a bypass hole. Hydraulic flow is delivered from the pump to an output chamber formed between a high-pressure end of the piston in the flow control cylinder and an output or combination fitting. Fluid from the output chamber passes through a bore in the fitting and on to the power steering system.
A pressure-sensing orifice in the fitting transfers the pressure of the fluid flowing to the steering system and delivers the pressure through a passage in the housing of the flow control assembly to a low-pressure chamber at the end of the piston. A compression spring in the low-pressure chamber forces the piston towards the fitting. A pressure force develops on the end of the piston due to the feedback pressure and combines with the spring force to keep the bypass port closed.
If the flow issuing from the pump outlet is too great and exceeds the force provided by the compression spring, the flow control piston will move away from the end of the fitting. As the piston moves away from the fitting, the piston partially or fully uncovers the bypass port to allow excess fluid to recirculate to the pump. When pressure demand increases, the spring forces the piston back toward the fitting to a new flow regulating position. Thus, the piston moves toward and away from the fitting in response to changes in the pressure system demand.
A problem often encountered with conventional flow control outlet fittings is undesirable fluctuations in fluid flow at high engine speeds. With the two-piece conventional fitting shown in FIG. 3, as the engine and pump speed increase, and the flow control piston moves to open the bypass valve, the forces acting on the front of the piston are unevenly applied and more fluid enters the bypass port than is desirable. Consequently, the fluid flow to the steering gear drops as shown at FIG. 8. As a result, there is an undesirable decrease in steering assist.
Another type of conventional flow control apparatus includes a fitting, shown in FIG. 4. This fitting 60 includes one end 62 that is in fluid communication with the pump outlet and faces the high-pressure end of the fluid control piston and a second end 64 through which fluid flows to the steering gear. The fitting 60 defines a bore 66 that runs the length of the fitting 60 that tapers with the direction of the fluid flow. With this type of fitting, as engine speed, and correspondingly pump speed increases, the flow of fluid to the steering gear rises and continues to rise. As a result, too much fluid flows to the steering gear and steering assist is undesirably increased.